A watertight switch comprises a cover and a base assembled together to form an accommodating space for mounting a switching module therein. The cover and the base, at the combination interface therebetween, respectively have a first junction interface and a second junction interface, which are inwardly shrunk to form a binding gap, the binding gap is gummed by an adhesive for fixedly combining and positioning the first and the second junction interfaces, and the adhesive will fill the binding gap without flowing over the first and the second junction interfaces.